The present invention relates to a nitride semiconductor device and a manufacturing method therefore, more specifically, it relates to a nitride semiconductor device having an electrode on the rear surface of a substrate.
Nitride semiconductors are used in light-emitting devices such as LED devices and laser devices, solar cells, passive light devices such as optical sensors, and electronic devices such as transistors and power devices.
LED devices using nitride semiconductors are widely used, for example, in traffic signal devices, large displays, light sources for backlights, and light sources for vehicles.
Laser devices using nitride semiconductors can be used as a light source for optical disks, light source for exposure, printers, optical communication systems, measurement and the like. Further, because their wavelengths oscillate within a wavelength region of 360 nm to 550 nm, laser devices comprising nitride semiconductors can be used as excitation light sources for biotech-related devices.
LED devices and laser devices using nitride semiconductors comprise a substrate made of sapphire or the like, on which are stacked, in the following order, an n-type nitride semiconductor layer, an active layer, and a p-type active layer, and a p-electrode is formed on the p-type nitride semiconductor layer.
Because a sapphire substrate has insulating properties, an electrode cannot be formed on the rear surface thereof. For this reason, by etching or otherwise removing the p-type nitride semiconductor layer and active layer from the nitride semiconductor layer stacking surface side, an n-electrode is formed on the n-type nitride semiconductor layer thus exposed. In other words, a p-electrode and n-electrode are formed on the stacking surface side of the semiconductor layer.
A sapphire substrate not only has a different lattice constant and coefficient of thermal expansion from the nitride semiconductor layer grown thereupon, it also has no cleavage. Therefore, it has been reported that the sapphire substrate is removed by polishing and the like because it is difficult to separate by cleavage nitride semiconductor layers including a sapphire substrate into devices. In other words, after the sapphire substrate is removed, the exposed nitride semiconductor layer is caused to function as the substrate. Because such a nitride semiconductor layer contains n-type dopants, an n-electrode can be formed on the surface opposite the growth surface of the nitride semiconductor layer. Thus, a p-electrode and n-electrode are disposed so as to oppose each other across a semiconductor layer.
With such a configuration, chip size can be made smaller than a configuration where a p-electrode and n-electrode are disposed on the semiconductor stacking surface side, thus improving yield.
To cause such a nitride semiconductor layer to function as a substrate, there is need for a certain film thickness, and in light of durability and other device properties, low dislocation is also needed. In order to achieve low dislocation for such a nitride semiconductor substrate, first, a mask layer is formed on a prescribed portion of the sapphire substrate. With the mask layer as a selective growth mask, from the mask opening an n-type nitride semiconductor layer is grown on the mask layer in the lateral direction. Thereafter, the sapphire substrate and mask are removed, resulting in an n-type nitride semiconductor layer substrate (e.g., JP-H11-214798-A).
Even with a nitride semiconductor substrate given low dislocation with such a method, dislocation concentration regions exist within the substrate. Because this dislocation is propagated to the nitride semiconductor layer grown on the substrate surface, dislocation concentration regions also exist in the nitride semiconductor layer of divided nitride semiconductor devices. For this reason, regions caused to function as a nitride semiconductor layer active layer and a light-emitting layer must avoid the dislocation concentration regions, and must be formed separated to a certain degree.
Further, in order to avoid having damage from device separation (as in division of wafer into bars or chips) extend to a region caused to function as an active layer or light-emitting layer, it is necessary to position a region caused to function as an active layer or light-emitting layer on a device inner side, so that the dislocation concentration regions are disposed on the device outer side.
However, dislocation concentration regions existing within a substrate do not necessarily exist in a consistent manner. That is, even if dislocation concentration regions exist in a striped shape on the top surface of a substrate main surface, they do not exist in a consecutive manner; for example, as shown in FIG. 5, there is a partially disconnected region 13 and a region 14 having a partially wide stripe shape. Thus, even if, as shown in FIG. 11, a rectangular electrode 51 is formed on a rear surface 101 of a substrate, when devices are divided into chips, the chip forming direction is not uniform in the dislocation concentration regions, the cleaving line extends in unexpected directions along the chip side surface, leading to chips that are partially broken (FIG. 12) or, because the cleaving direction bends, chips where the active layer region or light-emitting regions extend to the cleaving line (FIG. 13).
It is difficult to stably divide into chips nitride semiconductor devices grown on substrates in which such irregular dislocation concentration regions exist.